Sandwich panel for sound absorption

ABSTRACT

An aircraft cabin panel for sound absorption, with a core layer that comprises a plurality of cells that extend in an open manner across the thickness of the core layer and that are separated from each other by cell walls, and with a first cover layer that faces away from the sound field, as well as a second cover layer that faces towards the sound field and that comprises a plurality of perforation holes. The second cover layer is multi-layered and apart from a layer with the perforation also comprises a second layer that acts as a flow resistance device. The second layer is arranged on the outside, and visually closes off the perforation, and the outer layer comprises an air-permeable woven fabric.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/007,947 filed Dec. 17, 2007, the disclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a aircraft cabin panel for sound absorption. The invention in particular also relates to a device for sound absorption in an aircraft cabin.

The sound pressure level in a room, for example in an aircraft cabin, among other things depends on the extent of airborne sound absorption in a room. The higher the absorption of the walls or of the objects situated in the room, the lower the sound pressure level and thus the noise load in the room. Sandwich panels are used in rooms in order to improve the acoustic characteristics of the room, in that, for example, in aircraft cabins the sound that is perceived to be disturbing is absorbed by the panels. As the demand for greater comfort in rooms generally increases, at the same time there is a parallel demand for improved conditions of travel in aircraft, among other things also relating to room acoustics during flight operation. To this effect it is desirable to use sandwich panels for sound absorption in the interior of aircraft cabins so as to reduce the noise load in the cabin interior. The noises that are to be damped include, for example, the sound generated by the engines and also the sound generated by mechanical air conditioning, and last but not least the sound generated by passengers themselves.

BRIEF SUMMARY OF THE INVENTION

In relation with acoustic panels, sandwich constructions, for example, comprise a core layer with a multitude of cells that extend in an open manner across the thickness of the core layer and that are separated from each other by cell walls, and a first cover layer that faces away from the sound field, as well as a second cover layer that faces towards the sound field and that comprises a multitude of perforation holes. One option of increasing the absorption in the aircraft cabin relates to the use of sound-absorbent sandwich panels in which at least the cover layer that faces the exciting sound field is designed so as to be acoustically transparent. From EP 0 747 547 B1 an acoustic panel is known in which a core layer with a honeycomb structure is arranged between two cover layers, wherein both cover layers are designed so as to be multi-layered, with the top cover layer being perforated. Furthermore, DE 198 04 718 A1 shows a sandwich element in which a honeycomb-shaped core is arranged between two cover layers. The cover layer facing away from the sound comprises a closed layer of a fibre reinforced material. The front cover layer comprises a fibre reinforced net that is covered by a flexible foil or film. U.S. Pat. No. 4,671,841 discloses an acoustic element with a honeycomb core and a closed back. At the front a triaxial open-weave face sheet is bonded to the honeycomb core, which face sheet on its outer surface in turn is covered by a soft microporous sheet such as a stainless steel woven wire fabric. An acoustic liner with a closed back, a honeycomb core and a cover comprising two different woven fabrics at the front is also known from EP 0 747 285 B1. The outer woven fabric is exclusively decorative, whereas the more coarsely-meshed woven fabric between the outer fabric and the honeycomb core contributes to the stability of the acoustic panel. Finally, from DE 200 16 051 U1 a sound-absorbent composite panel is known in which a honeycomb core is arranged between two metal cover layers, of which the side facing the sound comprises microperforation. However, these composite panels are far too heavy for the use in an aircraft. The known sound-absorbent sandwich panels with a honeycomb core are associated with a disadvantage in that the achievable absorption in the woven fabric solutions is inadequate with a view to the height and width of the absorption spectrum. Perforated liner resonators as known from the above-mentioned DE 200 16 051 U1 excel in particular in the microperforated cover layers by broad bandwidth absorption behaviour. However, the inhomogeneous surface resulting from the perforation is a disadvantage. This is not acceptable for design reasons because the perforated surface would be visible in an application in the aircraft cabin. However, in order to damp the sound field within the passenger cabin the perforated surface must be situated on the face of the sandwich panel, which face points towards the cabin. However, it has been shown that the known sandwich panels do not meet the stringent optical requirements that are to be met by lining elements for passenger cabins of means of transport.

There may be a need to provide a sandwich panel for use in aircraft with improved sound absorption characteristics, which sandwich panel at the same time meets the optical requirements to be met by lining elements.

According to an exemplary embodiment of the invention, an aircraft cabin panel with a sandwich construction of the type mentioned above is provided in which the second cover layer is multi-layered and which apart from a layer with the perforation also comprises a second layer that acts as a flow resistance device.

The present invention is associated with an advantage in that the second layer can assume optical functions so that the panel is of excellent visual surface quality, thus meeting the standards required in aircraft engineering. In addition the layer acts as a flow resistance device, and, if designed properly, additionally improves the absorption characteristics of the panel.

The cells of the core layer are preferably tubular or honeycomb-shaped. Known honeycomb core structures are just as suitable as are tubular cell structures, in particular because they provide a good ratio of rigidity to weight, and furthermore are economical to produce. Apart from this, other core structures with cell walls that extend parallel are of course also suitable for the panel according to the invention. However, the core structures can also comprise diverging cell walls, i.e. conical cell walls or curved or folded cell walls. In principle any cell structures are suitable in which the cell walls extend from one cover layer to the other cover layer.

In a preferred embodiment the second layer is arranged on the inside of the layer with the perforation, wherein the second layer on its side facing towards the layer with the perforation comprises a light colour at least in the region of the perforation. As a result of this, passengers no longer perceive the perforation as black dots. Because of the light-coloured backing, said black dots disappear in favour of a calm surface design.

In a particularly preferred alternative embodiment the second layer is arranged on the outside, and visually closes off the perforation. As a result of this the outer layer defines the visual appearance, and the perforation of the perforated liner cannot be seen. The outer layer acts as a flow resistance device in front of the perforated absorber, and if correctly designed, said outer layer additionally increases the absorption characteristics of said absorber.

In a further preferred embodiment the outer layer comprises an air-permeable woven fabric. As a result of this the sandwich panel according to the invention is in a position to provide high-grade surfaces. The air permeability supports an optimum effect of the resonator or absorber arranged behind. The use of woven fabric is associated with an advantage in that all the layers with different flow resistance are more easily available, in other words adapting and matching the absorption characteristics is possible easily and cost-effectively. Of course, the panel can be washable for use on surfaces that are subject to particular wear, for example in the door region or the aisle region. To this effect the fabric has to be furnished accordingly, or a corresponding washable fabric is to be provided.

Preferably, the cover layer perforation pattern comprises a distance between holes that is shorter than the opening width of the honeycomb-like core structure so that for each honeycomb volume at least one perforation aperture, preferably several perforation apertures, is/are available.

In a particularly preferred embodiment the perforated layer is designed so as to be multi-layered, comprising a first layer that determines the strength, and a perforated film or foil as the second layer. Firstly, this is associated with more precise and simpler production of the perforation because the latter is significantly easier to achieve in a (thinner) film or foil than in a (thicker) supporting layer. Secondly, with a small perforation diameter the thinner film or foil also results in a smaller air volume within the aperture, in other words the mass of the air plug in the region of the perforation aperture is smaller, which in general corresponds to the design rule in the case of shallow design depths according to the selection of a small acoustic mass. A film or foil is particularly suitable for microperforated cover layers which are perforated surfaces with hole diameters less than 1 mm and a percentile of perforated surface of less than 1%. Depending on the design of the holes in conjunction with the volume behind the surface, these surfaces absorb the impinging sound in different frequency ranges as a result of a resonance effect and the damping inherent in the perforation. The design, also referred to as microperforation, provides a comparatively broad bandwidth absorption spectrum, which is particularly suitable for use in an aircraft because the sound to be damped in this application can comprise a large spectral frequency distribution. Microperforation is also suitable for aircraft use in that it does not take up any space nor does it involve any additional weight. The option of using pre-perforated semi-finished products provides a further advantage.

Preferably, the first layer is a grid-like or lattice prepreg as commonly used in aircraft cabin construction in order to be able to realise the construction with the lightest possible weight.

Further preferred is an embodiment in which the hollow spaces of the core layer differ in height so that by different volumes it can form different resonators, i.e. regions that have different absorption effects.

Preferred is an embodiment in which the first cover layer is designed so as to be closed. In this way the sound-absorbent sandwich panels according to the invention can in principle be used as finishing wall elements which do not require a separate cover for the back.

Preferably, the first cover layer is also designed so as to be acoustically transparent in order to be able to carry out additional sound reduction measures on the side facing away from the room, e.g. to achieve absorption in the broadest possible bandwidth with the application of sound-absorbent materials. To this effect a further preferred embodiment provides for a porous sound absorber on the outside of the first cover layer. The first cover layer, i.e. the face pointing away from the cabin, can, for example, comprise a lattice prepreg.

Preferred is an embodiment in which further layers are provided between the first cover layer and the second cover layer. In this way the constructive characteristics such as resistance to warping or deflection can be improved. By the further layers it is also possible to improve the acoustic characteristics.

In order to still further improve the sound insulation characteristics, in one embodiment at least some of the hollow spaces of the core layer are filled with a porous sound-absorbent material. This is also used to achieve absorption in the broadest possible bandwidth. This is expedient in particularly critical regions such as the aisle region or in first class.

In a particularly preferred embodiment the cell walls are perforated so that they are acoustically transparent in the direction parallel to the cover layers so as to allow the best possible utilisation of the adjacent hollow honeycomb spaces. This is advantageous in a perforation geometry that comprises a distance between holes, which distance exceeds the opening width of the cells of the core layer. In this arrangement the cell walls are to be designed such that they allow unhindered passage of sound between the cells. As a result of this a larger hollow space volume is available for sound absorption because the individual honeycomb cells communicate with each other by way of the apertures in the cell walls. As a result of enlarging the hollow space volume for each aperture of the perforation the resonance frequency is displaced to lower frequencies, i.e. to the range that is of particular interest in relation to absorption in the cabin. With a given thickness of the panel, the distance between the openings determines the hollow space volume. Perforation is at the same time also associated with reduced weight, which is always advantageous in aircraft engineering.

In a further preferred exemplary embodiment, the cover layers comprise fibre reinforced materials so that the lightest-possible and at the same time stable panels are obtained, wherein the mechanical and optical or visual characteristics of said panels correspond to those of the remaining panels in the cabin lining arrangement.

The invention relates in particular also to a device for sound absorption in an aircraft cabin in which between the aircraft supporting structure and the cabin space an interior lining arrangement formed by panels is provided. In order to improve noise reduction, according to the invention, within the cabin at least some of the panels are designed as sound-absorbent sandwich panels according to any one of the preceding embodiments, wherein particularly the surface of the sandwich panel can be made to match the remaining cabin layout.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, an exemplary embodiment of the invention is illustrated in more detail with reference to the enclosed drawings. The following are shown:

FIG. 1 a section through a sandwich panel with a perforated cover layer and a flow resistance device arranged on the outside;

FIG. 2 a section through a sandwich panel with a perforated cover layer in a multi-layered construction, and a flow resistance device arranged on the outside; and

FIG. 3 a comparison of the absorption coefficients of a panel according to prior art and of a panel according to the invention comprising a flow resistance device arranged on the outside.

DETAILED DESCRIPTION

FIG. 1 shows a section of a sandwich panel 10 according to the invention, with said sandwich panel 10 comprising a first cover layer 12 and a second cover layer 14, between which a core layer 16 in the form of a honeycomb core is arranged. The honeycomb core comprises a multitude of tube-like or honeycomb-like cells 18 that extend in an open manner across the thickness of the core layer 16 and that are uniform in design and are separated from each other by cell walls 20. The second cover layer faces towards the sound field and comprises a multitude of holes 22.

In order to create a visually pleasing appearance and at the same time for improvement or adjustability of the acoustic effect of the panel 10, the second cover layer 14 comprises several layers and, apart from a layer 24 with the perforation in the form of the holes (pores) 22, also comprises an outer layer 26, which acts as a flow resistance device, which outer layer 26 visually closes the perforation 22 and comprises an air-permeable woven fabric.

In a preferred embodiment, which is shown in FIG. 2, the perforated layer 24 is a multi-layered construction. A first layer, which determines the strength, of the perforated layer 24 is formed by a lattice prepreg 28. On the lattice prepreg 28 a perforated film or foil 30 is arranged, which in FIG. 2 is shown with an abstract thickness which, however, in reality is considerably thinner in relation to the thickness of the lattice prepreg 28. Preferably, the diameter of the holes 22 of the second cover layer 14 is less than 1 mm, and the percentile of perforated surface is less than 1%. This design, also known as microperforation, provides an absorption spectrum with a comparatively broad bandwidth, which is particularly suitable for use in aircraft because the sounds that are to be damped in this environment can comprise a wide spectral frequency distribution. The cover layers 12, 14 essentially comprise fibre reinforced materials.

The diagram in FIG. 3 (impedance tube measurement) shows the influence which a flow resistance device arranged on the outside of the microperforated second cover layer 14 has on the absorption behaviour of the perforated honeycomb liner. The vertical axis shows the absorption coefficient, while the horizontal axis shows the frequency (Hz). The lower curve PP (perforated panel) shows the values of a microperforated sandwich panel without a woven fabric layer as a flow resistance device arranged on the outside. The upper curve PP+F (perforated panel+fabric) shows the values relating to a sandwich panel according to the invention with a woven fabric layer as a flow resistance device arranged on the outside. In both cases the honeycomb core depth D is 15 mm. The diagram clearly shows the greater absorption coefficient of the panel according to the invention over almost the entire frequency spectrum.

Due to its light weight while at the same time providing an optimal absorption effect and adequate rigidity, the sandwich panel is, in particular, suited for use as a device for sound absorption in an aircraft cabin. To this effect between the aircraft supporting structure and the cabin space an interior lining arrangement comprising panels is provided, in which interior lining arrangement at least some of the panels are designed as sound-absorbent sandwich panels according to FIG. 1 or 2.

It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined.

It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims. 

1. An aircraft cabin panel for sound absorption, comprising: a core layer that comprises a plurality of cells that extend in an open manner across the thickness of the core layer and that are separated from each other by cell walls, and comprising a first cover layer that faces away from the sound field, and a second cover layer that faces towards the sound field and that comprises a plurality of perforation holes, wherein the second cover layer is multi-layered and wherein the second cover layer apart from a layer with the perforation also comprises a second layer that acts as a flow resistance device; wherein the second layer is arranged on the outside, and visually closes off the perforation; and wherein the outer layer comprises an air-permeable woven fabric.
 2. The aircraft cabin panel of claim 1, wherein the cells of the core layer are at least one of tubular or honeycomb-shaped.
 3. The aircraft cabin panel of claim 1, wherein the cover layer perforation comprises a distance between holes that is shorter than the opening width of the honeycomb-like core structure.
 4. The aircraft cabin panel of claim 1, wherein the perforated layer is multi-layered, and comprises a first layer that determines the strength, and a second layer comprising at least one of a perforated film or foil.
 5. The aircraft cabin panel of claim 4, wherein the first layer comprises a lattice prepreg.
 6. The aircraft cabin panel of claim 1, wherein the hollow spaces of the core layer differ in height.
 7. The aircraft cabin panel of claim 1, wherein further layers are provided between the first cover layer and the second cover layer.
 8. The aircraft cabin panel of claim 1, wherein at least some of the hollow spaces of the core layer are filled with a porous sound-absorbent material.
 9. The aircraft cabin panel of claim 1, wherein the cell walls are perforated so that they are acoustically transparent in the direction parallel to the cover layers.
 10. The aircraft cabin panel of claim 1, wherein the cover layers comprise fibre reinforced materials.
 11. A device for sound absorption in an aircraft cabin in which between an aircraft supporting structure and a cabin space an interior lining arrangement formed by panels is provided, wherein at least some of the panels comprise sound-absorbent aircraft cabin panels comprising: a core layer that comprises a plurality of cells that extend in an open manner across the thickness of the core layer and that are separated from each other by cell walls, and comprising a first cover layer that faces away from the sound field, and a second cover layer that faces towards the sound field and that comprises a plurality of perforation holes, wherein the second cover layer is multi-layered and wherein the second cover layer apart from a layer with the perforation also comprises a second layer that acts as a flow resistance device; wherein the second layer is arranged on the outside, and visually closes off the perforation; and wherein the outer layer comprises an air-permeable woven fabric. 